blog/content/posts/2024-06-24-union-find/index.md

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2024-06-25 00:01:48 +02:00
---
title: "Union Find"
date: 2024-06-24T21:07:49+01:00
draft: false # I don't care for draft mode, git has branches for that
description: "My favorite data structure"
tags:
- algorithms
- data structures
- python
categories:
- programming
series:
- Lesser known algorithms and data structures
favorite: false
disable_feed: false
---
To kickoff the [series] of posts about criminally underrated algorithms and data
structures, I will be talking about my favorite one: the [_Disjoint Set_][wiki].
Also known as the _Union-Find_ data structure, so named because of its two main
operations: `ds.union(lhs, rhs)` and `ds.find(elem)`.
[wiki]: https://en.wikipedia.org/wiki/Disjoint-set_data_structure
[series]: {{< ref "/series/lesser-known-algorithms-and-data-structures/">}}
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2024-06-25 00:02:08 +02:00
## What does it do?
The _Union-Find_ data structure allows one to store a collection of sets of
elements, with operations for adding new sets, merging two sets into one, and
finding the representative member of a set. Not only does it do all that, but it
does it in almost constant (amortized) time!
Here is a small motivating example for using the _Disjoint Set_ data structure:
```python
def connected_components(graph: Graph) -> list[set[Node]]:
# Initialize the disjoint set so that each node is in its own set
ds: DisjointSet[Node] = DisjointSet(graph.nodes)
# Each edge is a connection, merge both sides into the same set
for (start, dest) in graph.edges:
ds.union(start, dest)
# Connected components share the same (arbitrary) root
components: dict[Node, set[Node]] = defaultdict(set)
for n in graph.nodes:
components[ds.find(n)].add(n)
# Return a list of disjoint sets corresponding to each connected component
return list(components.values())
```